To invert the operator (REF ) and take care about its
possible degeneracy, consider the operators called Barnes-Rivers
projectors [1]}, [2]}. The starting point is to formulate
the projectors to the transverse and longitudinal subspaces of the
vector space. In the momentum representation we have
\(\omega _{\mu \nu } = \frac{k_\mu k_\nu }{k^2},\qquad \theta _{\mu \nu } = \eta _{\mu \nu } - \omega _{\mu \nu },\) 
